1. Field of the Invention
This invention relates to a method for the recovery of precious metal values from ores, concentrates, tailings and other products from mining and metallurgical operations. More particularly, this invention relates to a method of treatment of complex metal ores to enable the recovery of the precious metal values. The term "precious metals" to which this invention is primarily applicable, is generally understood to include, gold, silver, platinum and the metals of the platinum group, including osmium, ruthenium, iridium, palladium, and rhodium. In accordance with this invention these metals may be extracted from ores in percentages heretofore impossible or not economically advantageous.
2. Description of the Prior Art
It is well known that the precious metals such as gold, silver and platinum occur very often in the natural state, tied up in very complex ores. Often, for example, the precious metals occur in complexes either singly or in combination with iron, selenium, tellurium and other elements. These have proven extremely difficult to separate. Most of the standard mineral recovery processes do not work very well and result in uneconomic yields.
Presently, precious metals are most often recovered from their ores by smelting or through a cyaniding process whereby the ore is treated with cyanide compounds usually in an alkaline medium to complex out the precious metals. This is generally followed by separation of the precious metal bearing cyanide solution and precipitation of the cyanides or by addition of a carbonaceous, sorbent material followed by flotation and reduction of the cyanide concentrate.
Cyanidation processes have several inherent disadvantages, not the least of which is the poisonous nature of cyanide compounds and the accompanying difficulty of working with them. Further, the flotation process is not a particularly efficient method for the collection of precious metals and requires expensive equipment. Nor, when many precious metals occur together in the same ore is the separation of the cyanides an easy matter. Smelting, on the other hand, has the disadvantage that many of the precious metal values are lost, probably through escape through the stack. In any case, the prior art methods have never been economically or practically feasible when the ores are complex or do not contain the desired metals in large amounts. Recovery percentages are generally low.
It has long been well known that aqua regia is a solvent for most of the precious metals and leaching with aqua regia has long been used in the extraction of precious metals, particularly gold, silver and platinum from their ores. However, using ordinary aqua regia is not feasible with a complex ore because even heated aqua regia is not a particularly good solvent for complex ores. Further, while it was known that the hotter the aqua regia, the more efficient the extraction process would be, ordinary aqua regia boils at too low temperature to be serviceable. Still further, extraction of a complex ore with aqua regia yields a complex solution of chlorides and oxides which is extremely difficult to separate.
Those concerned with the separation of precious and noble metals, particularly gold, silver and platinum from complex ores, have long been desirous of a relatively simple, economically feasible process for the recovery of these metals. Especially, a process has been desired that did not have the dangers and disadvantages of cyanidation and the inefficiency associated with extraction with aqua regia.
It was reported in British Pat. No. 22,549 to recover precious metals from ores by combining a halide salt, nitrate salt, refractory metalloid and a free supply of air. The yield of recovered precious metal values using that technique is quite low and does not even rival more conventional techniques.